Grounds mostly for keeping abundant supply in bar (9)
I believe the answer is:
estaminet
'bar' is the definition.
(I have seen 'small bar' mean 'estaminet' so perhaps 'bar' could also mean 'estaminet')
'grounds mostly for keeping abundant supply' is the wordplay.
'grounds' becomes 'esta' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'mostly' means to remove the last letter.
'for' becomes 'to' (eg both can mean 'in the opinion of').
'keeping' indicates putting letters inside.
'abundant supply' becomes 'mine' (a mine is a large supply of something).
'to' with its final letter taken off is 't'.
'esta'+'t'='estat'
'estat' enclosing 'mine' is 'ESTAMINET'.
'in' is the link.
(Other definitions for estaminet that I've seen before include "Neat items (anag) - small bar or cafe" , "provides food and drink" , "Small French cafe" , "bistro" , "French restaurant" .)